Hydrogen peroxide and ferric ions

This idea has been attractive because of the similarity of such a scheme to an enzyme reaction, the catalyst representing the enzyme and the intermediate compound being analogous to the enzyme-substrate complex. The viewpoint has been taken by Spitalsky and co-workers (1) who have provided much evidence in support of it. The formation of various peroxide intermediates in many catalytic systems, e.g., chromate and molybdate, is easily demonstrated and to take a less obvious case, the compound FeH02++ recently shown to be present in mixtures of ferric ion and hydrogen peroxide (2), might be considered as an intermediate in the catalytic decomposition which occurs in this system. 

This brings out also the difference between the ferrous and the ferric reactions starting from ferric ions and hydrogen peroxide there must be an induction period during which the reduction process, ferric —> fer-... 

The evidence for the free radical mechanisms of the reaction between ferrous and ferric ions and hydrogen peroxide is fully discussed in the article by J. H. Baxendale in this volume, and it is necessary here only to summarize and comment on those features especially relevant to hemoprotein reactions. This evidence is essentially indirect. Experiment shows very reactive intermediates to be present and extensive kinetic studies reveal competition reactions for these intermediates in that the overall order of the reaction is found to depend on the reactant concentrations. A free radical mechanism is adopted because it accounts for the chemical reactivity of the system in the oxidation of substrates (Fenton s reaction) and the initiation of the polymerization of vinyl compounds (Baxendale, Evans, and Park, 84) and it provides a set of reactions which largely account for the observed kinetics. The set of reactions which fit best the most recent experimental data is that proposed by Barb, Baxendale, George, and Hargrave (83) ... 

The use of ferric ion and hydrogen peroxide as an oxidizing system has been very little studied. This is surprising because it provides the simplest model for peroxidase. Walton and Christiansen (94) showed that ethyl alcohol could be oxidized at 30°C. and the reaction was retarded in more acid solutions. The apparently low catalytic activity of this system would appear to be due to the slowness of the initiating... 

Iwahashi, H., Morishita, H., Ishii, T., Sugata, R. and Kido, R. (1989) Enhancement by catechols of hydroxyl-radical formation in the presence of ferric ions and hydrogen peroxide J. Biochem. 105 429-434. 

Such a reaction regenerates ferrous ion showing that only low concentration of Fe " is needed in the system [86]. Moreover, the photo-Fenton process may proceed using photons of wavelength close to 500 nm in the case of mixtures of ferric ion and hydrogen peroxide. It can also be performed by solar irradiation making it a low cost process [79, 80, 87, 88]. The photocatalytic cycle may be presented as shown in Scheme 6.6. 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

Merz and Waters (1949) showed that oxidation of organic compounds by Fenton s reagent could proceed by chain as well as non-chain mechanisms, which was later confirmed by Ingles (1972). Kremer (1962) studied the effect of ferric ions on hydrogen peroxide decomposition for Fenton s reagent. It was confirmed that once ferric ions are produced the ferric-ferric system is catalytic in nature, which accounts for relatively constant concentration of ferrous ion in solutions. 

Barb, W.G., Baxendale, J.H., George, P., and Hargrave, K.R., Reactions of ferrous and ferric ions with hydrogen peroxide, Trans. Faraday Soc., 47, 591, 1951. 

The investigation of Haber and Weiss (12) concerning the reaction of ferrous ion and ferric ion with hydrogen peroxide can hardly be omitted from any review in this field. The decomposition of hydrogen peroxide was interpreted to proceed according to the scheme... 

Polarographic catalytic currents" are well known, especially in inorganic electrochemistry, with classic examples, such as the reoxidation of ferrous ions—fonned cath-odically from ferric ions—by hydrogen peroxide or hydroxylamine. However, it appears obvious that the term catalysis is used too often quite ambiguously. Therefore it use in organic electrochemistry needs to be clarified a distinction should be made between the two main kinds of catalysis. This differentiation has been emphasized by Andrieux and Saveant [1,2]. 

In 1977, Kellogg and Fridovich [28] showed that superoxide produced by the XO-acetaldehyde system initiated the oxidation of liposomes and hemolysis of erythrocytes. Lipid peroxidation was inhibited by SOD and catalase but not the hydroxyl radical scavenger mannitol. Gutteridge et al. [29] showed that the superoxide-generating system (aldehyde-XO) oxidized lipid micelles and decomposed deoxyribose. Superoxide and iron ions are apparently involved in the NADPH-dependent lipid peroxidation in human placental mitochondria [30], Ohyashiki and Nunomura [31] have found that the ferric ion-dependent lipid peroxidation of phospholipid liposomes was enhanced under acidic conditions (from pH 7.4 to 5.5). This reaction was inhibited by SOD, catalase, and hydroxyl radical scavengers. Ohyashiki and Nunomura suggested that superoxide, hydrogen peroxide, and hydroxyl radicals participate in the initiation of liposome oxidation. It has also been shown [32] that SOD inhibited the chain oxidation of methyl linoleate (but not methyl oleate) in phosphate buffer. 

This includes the oxidation of sulfite (56) and of hydrogen peroxide (45, 65), as well as the oxidation of organic ligands like acetylacetonate (16, 72). The reaction of chromic ions with hydrogen peroxide has been also claimed to proceed via Mechanism 5 (93). Analogous to the behavior of ferric ions (56), the reduction of Cu+2 by sulfite ions (89) very likely proceeds by... 

Photoreduction of ferric ion. Lunck and co-workers observed the enhanced rate of photo-oxidation of salicyclic acid by hydrogen peroxide in the presence of Fe(III) as well as the increased rate of photodecomposition of hydrogen peroxide in the presence of transition metal ions.23 The ferrous ion reacts with hydrogen peroxide, generating a second hydroxyl radical and ferric ion, and the cycle continues. 

There is additional information available concerning the influence of the formed polymer upon the rate of polymerization. Parts (15) presented percent polymerization-time curves for acrylonitrile. The results are presented in the form of graphs in Fig. 1 to Fig. 5 in (15). The five figures refer to the different values of hydrogen peroxide, concentration of ferric ions and pH of the solution in which the polymerizations were performed. The three curves in each figure refer to the three concentrations of acrylonitrile at the start of polymerization. They were 1.06M, 0.80M and 0.40M of acrylonitrile. 

Photoreactions that involve transition metal ions, complexes or compounds can usually be classified as (photo)redox (simultaneous oxidation and reduction) processes. A representative non-photoassisted catalytic system is Fenton s reagent that produces HO radicals on reaction of ferrous ions (Fe2 +) and hydrogen peroxide (Scheme 6.287a). Its photochemical counterpart is the photo-Fenton process,1527 in which ferric (Fe3 + ) complexes in aqueous solutions (absorbing over 300 nm) are reduced to ferrous ions (Scheme 6.287b). 

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